I understand that this has to do with the firewall on the remote (scanned) host, but the main reason I am trying to use NMAP is to see which ports on the remote host are open and whether it is at all ping-able.

Well, I can ping it using 'ping' but not using NMAP. I find this very confusing.

Furthermore, the iptables firewall on the remote host has SSH checked as a Trusted Service ("accessible from all hosts and networks"). Why didn't NMAP detect this?

UPDATE: It turns out that when I last scanned the CentOS 6 remote host, it was in sleep mode. When I woke it up and tried NMAP again, I received: